The field of this invention is in thin film photodiode fabrication and the method of mounting the photodiode to a cryogenic cold finger used in such a device as thermal imaging systems. At present, thin film photodiodes are in the experimental stage. Even though thin film photodiodes appear to be feasible for use with infrared imaging systems, a problem still exists in the method of mounting a thin film photodiode array on a cryogenic cold finger.
Thin film photodiodes formed by Schottky barrier metal contacts have a disadvantage in that if the metal used to form the Schottky barrier is of sufficient thickness to form a good electrical contact, it will not transmit radiation. Therefore, even though the thick metal used to form the Schottky barrier forms a good electrical contact (i.e. a sensitive photodiode), the thick metal forming the diode may also shield the active area from incoming radiation.
Previously, some semiconductor materials that form photodiodes were grown on transparent substrates and the photodiodes turned away from the radiation source and placed directly on the cold finger with the radiation coming through the transparent substrate. There are many disadvantages to a semiconductor photodiode mounting method such as this on a cold finger. First, the thermal and mechanical bonding is very difficult, resulting in thermal mismatch and damage between the range of room temperatures and the cryogenic temperatures. Such damages can permanently destroy the photodiodes. One electronic lead per photodiode detector must be brought out from under the mounted substrate without shorting or damaging the array of detectors.